Determines if the digests and encrypted messages created with a
specific salt generator will include (prepended) the unencrypted
salt itself, so that it can be used for matching and decryption
operations.

Generally, including the salt unencrypted in encryption results will
be mandatory for randomly generated salts, or for those generated in a
non-predictable manner.
Otherwise, digest matching and decryption operations will always fail.
For fixed salts, inclusion will be optional (and in fact undesirable
if we want to hide the salt value).

Returns:

whether the plain (unencrypted) salt has to be included in
encryption results or not.